The state-of-the-art electroluminescent (hereinafter EL) devices are known to have the problem of deterioration in brightness of the EL phosphor by moisture. Countermeasures taken against this problem include packaging an EL device in a moisture proof film such as a fluoropolymer film or a polymer film laminated with an oxide barrier coat as disclosed in JP-B-4-51571. However, the moisture proof films are expensive. Another approach is to form a coating layer of a moisture proof inorganic material on EL phosphor particles by fluidized bed coating technology as proposed in JP-A-11-204254. The problem of this technique is that coated EL phosphor particles suffer from reduction of luminescence efficiency by the action of oxygen, water vapor, heat, etc. during formation of the coating layer. A method of forming a coating layer on EL phosphor particles in a solution is disclosed in JP-A-2001-185358 and JP-A-2002-226845, which has a disadvantage, as compared with the above-mentioned fluidized bed coating, that the coating layer is inferior in continuity, resulting in a failure to exhibit sufficient moisture resistance.